1. Field of the Invention
The present invention relates to an image forming device for use in a copying machine, printer, plotter, facsimile device, or the like. More particularly, the invention relates to an image forming device of the type using an aperture electrode assembly.
2. Description of the Related Art
European Patent Publication (A1) No. 587 366 (corresponding to Japanese Laid-Open Patent Publication No. Hei-6-155798) discloses an image forming device of the type using an aperture electrode assembly. The aperture electrode assembly is formed with a plurality of apertures therein and toner passage through the apertures is controlled in accordance with a drive signal applied to respective control electrodes connected to the apertures. The toner particles selectively pass through the apertures to form an image on an image recording medium.
More specifically, as shown in FIG. 1, the image forming device of the type described above includes a toner carrying roller 114 rotatably disposed in confrontation with an image recording medium 120 with an aperture electrode assembly 101 intervened therebetween. The toner carrying roller 114 has its own axis extending in the direction perpendicular to the surface of FIG. 1. The aperture electrode assembly 101 is disposed so that its longitudinal direction extends in the direction in parallel with the axis of the toner carrying roller 114. A back electrode roller 122 is rotatably disposed in a position opposite the toner carrying roller 114 with respect to the recording medium 120. The axis of the back electrode roller 122 is also in parallel with the axis of the toner carrying roller 114.
The aperture electrode assembly 101 includes a 25 .mu.m thick electrical insulation sheet 102 made of polyimide. A plurality of apertures 106 are formed in the insulation sheet 102. Each aperture is 100 .mu.m in diameter. The apertures 106 are aligned in the longitudinal direction of the aperture electrode assembly 101. The aperture electrode assembly 101 further includes control electrodes 104. The control electrodes 104 are provided in one-to-one correspondence to the apertures on the insulation sheet 102 so as to surround the respective ones of the apertures. The control electrode 104 is made of a copper foil of 1 .mu.m thickness and has a width of 20 .mu.m. The control electrodes formed surface of the aperture electrode assembly 101 faces the image recording medium 120.
A toner reservoir 111 storing toner therein is disposed below the aperture electrode assembly 101. A toner conveying roller 112 and the toner carrying roller 114 are rotatably disposed interiorly of the toner reservoir 111. Rotation of a toner conveying roller 112 conveys toner toward the toner carrying roller 114 and electrically charged toner particles are released toward the image recording medium 120 through the apertures 106.
The image forming device includes a data control circuit 108 for applying control voltages to the control electrodes 104 to control toner travels toward the recording medium 120 through the apertures 106. The control voltage corresponds to an image signal.
FIG. 2 shows one example of the aperture electrode assembly 101. As shown therein, a pair of supporting plates 107A and 107B are bonded to the surface opposite the control electrode formed surface of the insulation sheet 102. The supporting plates 107A and 107B are bonded along first and second lines on the insulation sheet 102, respectively. The first and second lines are in parallel with each other and also in parallel with an aperture line on which the apertures 106 align. The first line is at one side of the aperture line and the second line is at the opposite side of the aperture line. The supporting plates 107A and 107B bonded to the aperture electrode assembly 101 are fixedly secured to the toner reservoir 111.
If the supporting plates 107A and 107B are obliquely fixed to the insulation sheet 102 or the fixing positions are offset from the first and second lines, then the insulation sheet 102 wrinkles when the supporting plates 107A and 107B are secured to the toner reservoir 111 or internal stress of the insulation sheet 102 becomes non-uniform.
More specifically, although it is desirable that the two supporting plates 107A and 107B be fixed to the insulation sheet 102 so as to be separated by a uniform distance L along the entire length of the insulation sheet 102, the supporting plates 107A and 107B are actually fixed no in parallel with each other. Actually the insulation sheet 102 is bonded to the supporting plates 107A and 107B so that the distance between the two mounting plates 107A and 107B at one end of the insulation sheet 102 is longer by dL than that at the opposite end of the insulation sheet 102. When such an aperture electrode assembly 101 is fixed to the toner reservoir 111, tensile force will be imparted on the insulation sheet 102 in the direction perpendicular to the longitudinal direction of the supporting plates 107A and 107B. The tensile force elongates the insulation sheet 102 and generates internal stress inside the insulation sheet 102.
FIG. 4 is a graphical representation indicating a relationship between elongated length of the insulation sheet 102 and internal stress thereof. The solid line in FIG. 4 is for the insulation sheet 102 with the supporting plates bonded in correct positions. The internal stress becomes T when the insulation sheet 102 elongates S. The dotted line in FIG. 4 is for the insulation sheet 102 with the mounting plates bonded in shifted positions. The widened portion of the insulation sheet 102 is less elongated by dL than the correctly bonded insulation sheet and the internal stress is lower by TB than T. Thus, the tensile force on the insulation sheet 102 is not uniform in the longitudinal direction of the toner carrying roller 14. Accordingly, the insulation sheet 102 wrinkles and the insulation sheet 102 is imparted with non-uniform internal stress. The wrinkles on the insulated sheet 102 and the non-uniform internal stress will affect the behavior of the toner particles under the apertures so as to move irregularly. As a result, the toner image density in the longitudinal direction of the toner carrying roller 114 will not be uniform, and clogging of image may partially occur.
FIG. 3 shows clamping of the insulation sheet 102. The insulation sheet 102 are clamped at both edge portions with two pairs of supporting plates 107A, 107C and 107B, 107D. The lower supporting plates 107A and 107B are secured to the toner reservoir 111 by an adhesive or screws. In such a clamping method, when driver IC chips are bonded by an adhesive to the surface of the insulation sheet 102, the rigidity of the IC chip mounted portions becomes high when compared with that of other portions, so that the insulation sheet 102 wrinkles. As a result, the toner passing amount in the respective apertures becomes non-uniform, and so the print quality of the reproduced image is degraded caused by the density variation of the reproduced image and image distortions.
FIG. 5 shows another conventional aperture electrode assembly 101. Conventionally, no concern has been made as to the direction in which the control electrodes 104 extend on the insulated sheet 102. If the control electrodes 106 have an obliquely arranged pattern as shown in FIG. 5, the central portion of the insulation sheet 102 is concentrated with the control electrodes 104. Therefore, the central portion of the insulation sheet 102 is high in Young's modulus, so strong against tensile strength. On the other hand, side portions of the insulation sheet 102 where no control electrodes are formed is low in Young's modulus, so weak against tensile strength. Non-uniform tensile strength over the entire surface of the insulation sheet 102 results in wrinkles of the insulated sheet 102. Further, abutting force of the toner carrying roller 114 against the insulation sheet 102 will not be uniform. This problem is particularly noticeable when the driver IC chips 109 are mounted on the surface of the insulation sheet 102 as shown in FIG. 5 where the pitch of the control electrodes 104 is narrowed toward the IC chips 109. The driver IC chips 109 control voltages applied to the respective control electrodes 104, and it is advantageous to mount the IC chips 109 on the insulation sheet 102 to reduce the manufacturing cost.